The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to a system for trimming marine seismic streamers by selectively adjusting the buoyancy of such streamers.
Seismic streamer cables are towed in the water behind marine seismic vessels. The vessels tow acoustic energy sources such as air guns to generate energy for penetrating subsurface geologic formations, and the streamers support hydrophones for detecting energy reflected from the subsurface formations. The streamers typically comprise hydrophone strings, other electrical conductors, and buoyancy material. A flexible jacket surrounds the streamer exterior to reduce frictional drag through the water, to prevent water intrusion, and to resist damage to the electrical conductors and buoyancy billets.
Typical streamer cables are three to eight kilometers in length and are towed below the water surface to avoid surface wave action and other environmental factors detrimental to the seismic operations. The elevation of the streamers is selected for the water conditions, depth, and for the desired seismic data requirements. Control over the streamer elevation is critical to the accuracy of the acoustic source energy generated and to the reflected signal reception by hydrophories attached to the streamers. The streamer elevation may be uniform or varied over the entire streamer length.
Buoyant seismic cables are identified in U.S. Pat. No. 3,795,759 to Rhyne (1974), wherein a plurality of inflatable buoyancy devices were attached to the cable casing and had gas transmission conduits for selectively providing gas through the cable to level individual buoyancy units. Another leveling concept was disclosed in U.S. Pat. No. 3,909,774 to Pavey (1974), wherein a pressure sensing switch and controller automatically controlled the flow of a buoyancy control liquid from a supply line to a streamer to increase buoyancy, and for discharging the control fluid to decrease streamer buoyancy. Other buoyant cables were disclosed in U.S. Pat. No. 4,496,796 to Matikainen et al. (1985) wherein heat from an interior conductor was transmitted to the outer cable sheath without passing through an interior cable float, and in U.S. Pat. No. 5,046,057 to Berniu (1991). wherein a flotation material was positioned around a central core member, electrical detectors were acoustically isolated from the core member, and the assembly was surrounded with an acoustically transparent material and an outer sheath material.
In U.S. Pat. No. 3,794,965 to Charske (1972), buoyancy of a marine seismic cable was controlled by the selective pumping of water into and out of flotation cells. A controller engaged with the flotation cells provided for continuous elevation adjustment of the cable system. In U.S. Pat. No. 4,709,355 to Woods et al. (1987), a computer monitored depth measurements and was capable of generating a signal for modifying the elevation of a diving body attached to the marine cable. In U.S. Pat. No. 4,745,583 to Motal (1988), the buoyancy of individual cable sections was controlled with a pump, elongated bladder, and buoyant fluid. Another device for leveling seismic marine cables was disclosed in U.S. Pat. No. 5,459,695 to Manison (1995), wherein a flotation tube within the seismic cable was selectively flooded to varying the seismic cable buoyancy. A longitudinal seam provided access to the streamer interior, and an environmentally safe gel filled the streamer interior. A reusable skin lock prevented water intrusion through the longitudinal seam into the streamer interior.
In U.S. Pat. No. 5,278,804 to Halvorsen (1994), detachable weights were connected to the outside surface of the streamer cable The outer shape of each weight were streamlined to reduce "noise" as the streamer and weights were towed through water. In U.S. Pat. No. 4,086,561 to Wooddy, Jr. (1978), individual weights were formed within heat shrinking tubing, which provided a retainer and a cover for the individual weights. The weight package was then attached to the streamer cable exterior with bands or straps. Other cable weights were disclosed in U.S. Pat. No. 4,953,146 to McMurray (1990), U.S. Pat. No. 3,287,691 to Savit (1964), U.S. Pat. No. 2,791,019 to Du Laney (1957), and in U.S. Pat. No. 2,570,707 to Parr, Jr. (1951),
Buoyancy for cables is typically provided by including a filling liquid or gel that has a density less than sea water, or by using plastic or glass microspheres embedded in a solid or semi-solid material. For solid core streamers, the buoyancy of each streamer section depends on the applicable manufacturing tolerances. In many circumstances, the streamer cable buoyancy can change over the streamer length due to varying manufacturing conditions. For example, buoyancy in a new streamer can range from three kilograms buoyancy per section to between ten and fifteen kilograms buoyancy per section. Manufacturing variances in the buoyancy per section can be caused by differences in billet sizes, in the adhesive materials binding individual components, and in other variables.
In addition to manufacturing variances, the buoyancy of streamer cables can become reduced over time due to different factors such as compression or "set" of the buoyant microspheres caused by variations in streamer cable depth. Mild degradation of section buoyancy is experienced due to the deployment and retrieval cycling of a seismic streamer. Larger and sometimes permanent buoyancy loss is experienced when cables dive to deeper depths to avoid surface obstacles and hazards. Catastrophic loss of buoyancy can happen when sections are accidentally driven to extreme depths. The sinking of the streamer cable can subject the streamer to extreme underwater pressures. Under such pressure, solid buoyancy streamers can assume a permanent set, and embedded microspheres partially or completely collapse and reduce the cable buoyancy. In extreme conditions, the streamer can become negatively buoyant from collapse of the microspheres.
Damage to a streamer or individual section length requires streamer replacement or repair. The entire billet material can be removed and replaced with a new buoyant material, however this process is expensive and cannot be readily performed in the field. Consequently, the damaged streamer section must be recovered from the water so that the streamer section can be replaced on-board the seismic vessel or at land based facilities.
In addition to manufacturing variances and buoyancy changes over time, seismic operations in different regions have different buoyancy requirements. Differences in temperature and salinity significantly affect streamer buoyancy, particularly for streamers filled with oil and other liquid and semi-liquid materials. Moving seismic operations from the North Sea to the Persian Gulf significantly changes the buoyancy performance of a streamer. Even within the same regions, variations in salinity across river deltas and tidal areas also significantly affects streamer buoyancy. Although streamers having solid buoyancy billets reduce the impact of temperature and salinity variations, buoyancy adjustments from along t he streamer are essential to the accurate collection of seismic data.
To adjust for quality control variations in the manufacture of streamer cables and other variations in the streamer buoyancy, external weights are typically positioned on the streamer exterior to tune the streamer buoyancy as described above. Such external streamer weights generate drag as the streamer is towed through the water and further generate signal noise detrimental to the quality of reflected signals detected by the hydrophones. Although the signal noise can be substantially removed with filtering techniques, signal noise adversely affects the c quality of detected data. In addition to these disadvantages of external streamer weight systems, the weights can scar and penetrate the exterior sheath protecting the streamer interior against water intrusion.
Accordingly, a need exists for an improved system for tuning the buoyancy of marine streamer cables, for adjusting to differing environmental conditions, and for easily accommodating natural loss of intrinsic buoyancy as the streamer sections age. The system should be capable of being implemented at the seismic survey site or the manufacturing facility, should permit fine adjustments to the marine streamer buoyancy, and should minimize disruption to seismic operations.